A static electricity protection diode is typically used for protecting a semiconductor element or a circuit from static electricity. A horizontal diode can be used as a static electricity protection diode where the demand for electrostatic discharge (ESD: Electrostatic Discharge) robustness is low, which is often the case with the input section of a typical IC. The horizontal diode has a low robustness because current only flows in the vicinity of the surface of the semiconductor substrate. A static electricity protection diode of a vertical type, however, can provide a higher ESD robustness with the same area because the electrical current flows in a vertical direction with respect to the semiconductor substrate.
FIG. 11 is an example of such vertical static electricity protection diodes for protecting an input circuit, which is applicable where a comparatively high ESD robustness value is demanded. In FIG. 11, a resistor 10 is inserted between a first static electricity protection diode 8 and a second static electricity protection diode 9, and is connected to an internal input circuit 11. In this case, a horizontal diode is often employed for the second static electricity protection diode 9, since it can keep the current flowing due to the resistor 10 low. Generally, the impedance of the input of the internal input circuit 11, which consists of a CMOS circuit or the like, is high since it is connected to the gate of a MOSFET. Thus, even if the resistor 10 is comparatively large, no great influence is exerted upon its characteristic.
FIG. 12 explains the operation of the circuit of FIG. 11, namely a graph 12 schematically showing the current (I)-voltage (V) characteristic of the first static electricity protection diode 8, a graph 13 schematically showing the I-V characteristic of a circuit in which the second static electricity protection diode 9 and the resistor 10 are connected in series, and a graph 14 schematically showing the I-V characteristic of the second static electricity protection diode 9. When a current Iz1 flows in the first static electricity protection diode 8, a voltage Vz1 is created, and a current Ir flows in the circuit consisting of the second static electricity protection diode 9 and the resistor 10 connected together. Accordingly, a voltage Vz2 is created in the second static electricity protection diode 9. As will be understood from the above, it is possible for the voltage Vz2 to approach the breakdown voltage of the static electricity protection diode 9 when the value of the resistor 10 high. Since the current Ir can be made small, it is possible to protect the internal input circuit 11 in an effective manner, even if the dynamic resistance of the first static electricity protection diode 8 and the second static electricity protection diode 9 are not so very low.
On the other hand, FIG. 13 is an example in which such a diode is used for protecting an output MOSFET of the open drain type. Here, the cathode and the anode of a static electricity protection diode 21 are respectively connected to the drain and the source of a MOSFET 20, which is the object of protection. While the portion of the I-V characteristic, which is higher than the avalanche voltage of a normal MOSFET, is like that shown by a graph 23 of FIG. 14, the voltage increases along with increase of the current, while exhibiting a negative resistance in a higher current region. This negative resistance is due to the negative resistance of a parasitic diode 22 of the MOSFET 20. Specifically, the current due to an avalanche in the parasitic diode becomes the base current of a parasitic transistor, and there are two modes of negative resistance due to the operation of this parasitic transistor. In particular, since the current concentration due to negative resistance can more easily occur in the former than the latter, it can more easily end up being destroyed.
To protect this type of MOSFET 20 from surge due to ESD or the like, it is necessary to bypass the current flowing in the MOSFET 20 with a protection element, so that the voltage applied to the MOSFET 20 does not exceed the voltage (breakover voltage) that enters into the negative resistance region. It is possible to protect the MOSFET 20 with a protection diode having the I-V characteristic shown by a graph 24 of FIG. 14 if the current is within I1 and I0. Moreover, with a protection diode having the I-V characteristic shown by a graph 25, it is possible to protect the MOSFET 20 if the current is within I2 and I0. However, it is also necessary to not exceed the breakover voltage with the MOSFET 20 in the ON state, since sometimes, due to the dV/dt of an ESD surge, the gate voltage of the MOSFET 20 can rise and exceed the threshold value (for the sake of brevity this is not shown in the figures precisely). The degree to which this gate voltage rises depends, not only upon the dV/dt of the surge, but also upon the gate wiring resistance of the MOSFET and its drive circuitry and so on. A vertical static electricity protection diode described above, with a comparatively low dynamic resistance can be obtained in the same area, is particularly effective for static electricity protection in this type of case.
FIG. 16 is an example of the cross sectional structure of a prior art vertical static electricity protection diode. Here, a p− layer 3 is formed by an epitaxial growth over a p++ substrate 1, an n cathode region 4 is formed by diffusion upon the surface of the p− layer 3, and an n+ contact region 5 is formed within this n cathode region 4, to obtain a low contact resistance. Moreover, due to the heat processing, which is performed to form the n cathode region 4 or required elements for an IC, diffusion takes place from the p++ substrate 1 into the p− layer 3, thus forming a p+ region 2. With a static electricity protection diode of this type of structure, it is simple and easy to form an IC using a p+/p− epitaxial substrate, but a certain thickness is needed for the p− layer 3, to prevent any influence being exerted upon the characteristics of the devices formed upon the surface, due to diffusion from the p+ substrate. For example, FIG. 15 shows a horizontal MOSFET 70 that can be applied as a high side switch, such as is widely used in the automobile field, with the RESURF condition where the net impurity amount of the n wells 71 per unit area (the impurity amount obtained by subtracting the impurity amount of the p− layer 3 from the impurity amount of the n wells) is about 1×1012 cm−2 (actually quite a high value is desirable to enhance the on breakdown voltage). When forming such a horizontal MOSFET 70, it is necessary for the depth of the n wells 71 to be greater than or equal to about 4 μm to achieve a punch-through voltage of greater than or equal to 40V, although this is also influenced by the depth of the p wells 72. In other words, it is necessary for the thickness of the p− layer 3 to be greater than or equal to about 4 μm to prevent the n wells 71 from becoming too shallow (i.e., thinning) due to the formation of the p+ region 2 resulting from diffusion from the p++ substrate 1, thus deteriorating the punch-through breakdown voltage.
Sections B and C of FIG. 16, respectively show, in correlation with the cross sectional structural view of section A of FIG. 16, an example of a density distribution in the depth direction of the static electricity protection diode of this type and an example of the electric field intensity distribution with a starting avalanche voltage. This type of static electricity protection diode is designed to have a somewhat lower breakdown voltage than the breakdown voltage of a normal MOSFET, presenting the possibility of destruction because of high current flowing in the MOSFET due to excessive voltage between the source and the drain of the MOSFET.
Even though this type of static electricity protection diode has a vertical type structure, it sometimes cannot provide sufficient protection to the device since its dynamic resistance is not sufficiently low. Also, the protection diode itself can be destroyed since it has negative resistance. Accordingly, there still remains a need for an improved protection diode, in particular, one that provides low dynamic resistance, without having negative resistance over a wide current region. The present invention addresses this need.